This invention relates in general to wireless communication systems, and more specifically to a method and apparatus in a zero intermediate frequency (ZIF) receiver for facilitating reception of a signal transmitted on one of a plurality of channels that can produce adjacent channel interference.
In a zero intermediate frequency (ZIF) receiver, DC offsets can appear at the receiver output, which land in the center of the received band, thus causing interference with the received signal. In some prior-art receivers, notching out the DC with a narrow highpass filter has provided acceptable performance. Unfortunately, this technique cannot be used when the signal contains a substantial amount of spectral energy at DC, such as is the case for a pilot symbol assisted quadrature amplitude modulated (PSA-QAM) signal. A known method of combating the DC offset problem is to mix the signal off center by one-half the channel spacing, so the DC notch appears between two channels. This, however, can cause problems when a strong interferer is present on an adjacent channel, due to imaging as a result of I/Q imbalance.
What is needed is a solution that can simultaneously solve the DC offset problem associated with zero-centered reception, and the adjacent channel interference problem caused by I/Q imbalance in a non-zero-centered system.
An aspect of the present invention is a method in a zero intermediate frequency (ZIF) receiver for facilitating reception of a desired signal transmitted on a desired channel of a plurality of channels that can produce adjacent channel interference. The method comprises the steps of detecting the desired signal; and downmixing the desired signal to baseband, off-centered by an amount, thereby generating a downmixed signal. The amount is determined by at least one of (a) whether pilot symbols are present in the desired signal, and (b) signal levels of the desired signal and adjacent channel signals.
Another aspect of the present invention is an apparatus in a zero intermediate frequency (ZIF) receiver for facilitating reception of a desired signal transmitted on a desired channel of a plurality of channels that can produce adjacent channel interference. The apparatus comprises a receiver front end for detecting the desired signal; and a downmixer for downmixing the desired signal to baseband, off-centered by an amount, thereby generating a downmixed signal. The amount is determined by at least one of (a) whether pilot symbols are present in the desired signal, and (b) signal levels of the desired signal and adjacent channel signals.
Another aspect of the present invention is a subscriber unit for facilitating reception of a desired signal transmitted on a desired channel of a plurality of channels that can produce adjacent channel interference. The subscriber unit comprises a receiver front end for detecting the desired signal, and a processing system coupled to the receiver front end for processing the desired signal and for controlling the subscriber unit. The subscriber unit further comprises a downmixer coupled to the receiver front end and coupled to the processing system for downmixing the desired signal to baseband, off-centered by an amount, thereby generating a downmixed signal. The amount is determined by at least one of (a) whether pilot symbols are present in the desired signal, and (b) signal levels of the desired signal and adjacent channel signals.